1. Field of the Invention
The present invention relates to a mounting board, a method for manufacturing the same, and a semiconductor module.
2. Description of the Related Art
The functionality and sophistication of portable electronics equipment such as cellular phones, personal digital assistants (PDAs), digital video cameras (DVCs), and digital still cameras (DSCs) has been advancing at increasingly high speed. Under the circumstances, miniaturization and weight savings have become essential in order for these products to continue to be accepted in the market. To achieve this, highly integrated system LSIs have been sought. In the meantime, enhanced usability and convenience have been desired of such electronics equipment, and functional and performance sophistication has been required of LSIs to be used for the equipment. Consequently, while LSI chips of higher integration have grown in the number of I/Os, the packages themselves have also required miniaturization strongly. For the sake of satisfying both the requirements, there has been a high demand to develop a semiconductor package suited to packaging semiconductor components on a board at high density. To meet such a demand for high density, circuit substrates for mounting LSI chips have recently been made finer and become multilayered. For example, a method has been proposed for manufacturing a multilayered circuit substrate in which a film circuit substrate (resin film) provided with fine wiring is pasted onto a core substrate (flexible circuit substrate) via an adhesion layer (adhesive layer). According to this method, it is possible to fabricate the film circuit substrate by using a different process suited to forming fine wiring, and to achieve a high yield by selecting and pasting conforming pieces individually.
FIG. 7 is a cross-sectional view schematically showing the structure of a conventional device mounting board. The conventional device mounting board (multilayered circuit substrate) comprises: a flexible circuit substrate 101 having a circuit pattern 103; a resin film 102 having a circuit pattern 104 formed on either one or both of its surfaces, along with projections 106 capable of electrical connection on one surface; and an adhesive layer 110 formed by curing a nonconductive adhesive paste between the flexible circuit substrate 101 and the resin film 102. The projections 106 are fixed by the adhesive layer 110, and establish electrical connection between the circuit pattern 103 on the flexible circuit substrate 101 and the circuit pattern 104 on the resin film 102.
In the conventional device mounting board described above, the adhesive layer 110 and the flexible circuit substrate 101 are made of different materials, however, and temperature changes can thus easily cause thermal stress at the interface between the two due to different coefficients of thermal expansion of their respective materials. Consequently, when the device mounting board is subjected to heat, exfoliation may occur at the joint areas between the adhesive layer 110 and the flexible circuit substrate 101. There has thus been the possibility of a drop in connection reliability between the circuit pattern 103 on the flexible circuit substrate 101 and the circuit pattern 104 on the resin film 102.
Moreover, when the resin film 102 and the flexible circuit substrate 101 are pasted together with the adhesive layer 110, the low flowability of the adhesive layer 110 causes the adhesive layer 110 to remain at the interface between the projections 106 on the resin film 102 and the circuit pattern 103 on the flexible circuit substrate 101. This can also lower the connection reliability between the circuit pattern 103 on the flexible circuit substrate 101 and the circuit pattern 104 on the resin film 102.